A method for preparing carbon nanofluid

ABSTRACT

The present invention provides a method for preparing a carbon nanofluid. The method includes providing a base fluid, providing a number of carbon nanotubes, combining the carbon nanotubes with the base fluid, dispersing the carbon nanotubes substantially evenly in the base fluid through a physical agitation operation, and cooling a system performing the physical agitation operation during the physical agitation operation. The present invention also provides a carbon nanofluid capable of serving as a heat transfer fluid. The carbon nanofluid includes about 99.8 to about 98% by volume of a base fluid, and from about 0.2 to about 2.0% by volume of functionalized carbon nanotubes substantially evenly-dispersed in the base fluid.

BACKGROUND OF THE INVENTION

The present invention relates to a carbon nanotechnology, more particularly to a method for preparing carbon nanofluid with enhanced thermal conductivity.

The thermal conductivity of heat transfer fluid plays an important role in the development of energy-efficient heat transfer equipment including electronics, heating, ventilating, air-conditioning, refrigeration, and transportation. Development of advanced heat transfer fluids is clearly essential to improve the effective heat transfer behavior of conventional heat transfer fluids. Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids required in many industrial applications.

U.S. Pat. No 5,863,455 to Segal disclosed a colloidal fluid having metallic particles in a carrier fluid to insulate and cool an electromagnetic device which generates heat as a result of utilizing high current densities and high alternative current (AC) voltages inside the electromagnetic device. A new class of heat transfer fluids has also been developed by suspending metal or metal oxide particles in liquids as disclosed in U.S. Pat. No. 6,221,275 to Choi et al. The metal or metal oxide particles are produced and dispersed in a vacuum while passing a thin film of the fluid near the heated substrate.

Emerging carbon nanotechnology shows promise in many aspects of engineering applications. Recently, carbon nanotubes have been proposed with growing popularity as a stable nanomaterial with enhanced thermal conductivity. However, carbon nanotubes are strong and flexible, yet very cohesive. This makes it difficult to evenly-disperse them into fluids for providing an efficient heat transfer agent in the energy management.

BRIEF SUMMARY OF THE INVENTION

One example of the invention provides a method for preparing a carbon nanofluid with enhanced thermal conductivity. The method comprises providing a base fluid, providing a number of carbon nanotubes, combining the carbon nanotubes with the base fluid, and dispersing the carbon nanotubes substantially evenly in the base fluid through a physical agitation operation, and cooling a system performing the physical operation during the physical agitation operation.

Another example of the invention provides a method for preparing a fluid capable of serving as a heat transfer agent. The method comprises introducing a number of functional groups onto carbon nanotubes for providing functionalized carbon nanotubes, providing a base fluid, combining the functionalized carbon nanotubes with the base fluid, and dispersing the carbon nanotubes substantially evenly in the base fluid through an ultrasonication operation, cooling a system performing the ultrasonication operation during the ultrasonication operation.

In a further another example, the present invention provides a carbon nanofluid capable of serving as a heat transfer fluid. The carbon nanofluid comprises about 99.8 to about 98% by volume of a base fluid and from about 0.2 to about 2.0% by volume of functionalized carbon nanotubes substantially evenly-dispersed in the base fluid, wherein the carbon nanofluid has a thermal conductivity at least 1.3 times higher than a base fluid having no carbon nanotubes.

In yet one other example, the present invention provides a carbon nanofluid made by the process of introducing a number of functional groups onto carbon nanotubes for providing functionalized carbon nanotubes, providing a base fluid, combining the functionalized carbon nanotubes with the base fluid, and dispersing the carbon nanotubes substantially evenly in the base fluid through an ultrasonication operation, and cooling a system performing the ultrasonication operation during the ultrasonication operation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

In the drawings:

FIG. 1 is a schematic diagram illustrating a laboratory apparatus for generating functionalized carbon nanotubes according to one embodiment of the invention;

FIG. 2 is a schematic diagram illustrating an ultrasonic homogenizer arranged adjacent to a dual tube heat exchange system according to another embodiment of the invention; and

FIG. 3 is a schematic diagram illustrating a dual tube heat exchange system according to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

The present invention provides a method for preparing a carbon nanofluid. The method comprises providing a base fluid, providing a number of carbon nanotubes, combining the carbon nanotubes with the base fluid, and dispersing the carbon nanotubes substantially evenly in the base fluid through a physical agitation operation, and cooling a system performing the physical agitation during the physical agitation operation.

The carbon nanotubes described in the present invention comprises at least one of single-walled, double-walled and multi-walled carbon nanotubes having a plurality of functional groups introduced thereon.

Therefore, the term “to functionalize” herein refers to introduce a plurality of functional groups to surface of the carbon nanomaterial by chemical modification such as acidic treatment for enhancing thermal conductivity and solubility of the carbon nanomaterial in the aqueous, inorganic or organic solutions.

In one embodiment of the invention, each of the functional groups comprising COOH is introduced by treating the carbon nanotubes with an acidic solution comprising at least one of H₂SO₄, HNO₃, HCl and CH₃COOH. The functionalized carbon nanotubes are combined with the base fluid and then dispersed substantially evenly in the base fluid through a physical agitation operation. During the physical agitation operation, a cooling operation may be applied for cooling a system performing the physical agitation operation during the physical agitation operation.

In another embodiment of the invention, the carbon nanotubes are surface-modified or functionalized by treating the carbon nanotubes with the acidic solution comprising H₂SO₄ and HNO₃. As a result, the functional group comprising COOH is introduced onto the surface of the carbon nanotubes. The functionalized carbon nanotubes may be further purified by subjecting to high speed centrifugation to separate the unbound acidic mixture from the functionalized carbon nanotubes. Next, the purified carbon nanotubes may be washed with the base fluid before being combined with the base fluid and being dispersed in the base fluid. The functionalized carbon nanotubes are combined with the base fluid and then dispersed in the base fluid through a physical agitation operation, such as magnetic force agitation or ultrasonication operation. A cooling operation is performed for cooling a system performing the physical agitation operation during the physical agitation operation.

In a further embodiment of the invention, the carbon nanotubes are surface-modified or functionalized by treating the carbon nanotubes with an acidic solution including H₂SO₄ and HNO₃ in a ratio of about 3:1. The functionalized carbon nanotubes may be further purified by subjecting to high speed centrifugation to separate the unbound acidic mixture from the functionalized carbon nanotubes. Next, the purified carbon nanotubes are washed with the base fluid before being combined with and being dispersed within the base fluid. The purified carbon nanotubes are then combined with the base fluid and dispersed in the base fluid through an ultrasonication operation. During the ultrasonication operation, a cooling operation may be applied to cool a system performing the ultrasonication operation. In accordance with one example, the ultrasonication operation may be carried out using an ultrasonic homogenizer with a cooling system arranged adjacent to the ultrasonic homogenizer for cooling the ultrasonic homogenizer

Although the carbon nanotubes are functionalized by an acidic solution as described in the above embodiments, it is noted that the present invention is not limited to functionalizing the carbon nanotubes using this particular technique. Other surface modification techniques which cause addition or introduction of functional groups on the carbon nanotubes may be adopted.

The invention also provides a method for preparing a fluid capable of serving as a heat transfer agent. The method comprises steps of introducing a number of functional groups onto carbon nanotubes for providing functionalized carbon nanotubes, providing a base fluid, combining the functionalized carbon nanotubes with the base fluid, and dispersing the functionalized carbon nanotubes substantially evenly in the base fluid through an ultrasonication operation, and cooling a system performing the ultrasonication operation during the ultrasonication operation.

Similarly, the carbon nanotubes, such as single-walled, double-walled or multi-walled carbon nanotubes are functionalized by treating the carbon nanotubes with an acid mixture including H₂SO₄ and HNO₃ in a ratio of about 3:1. The functionalized carbon nanotubes may be further purified by high speed centrifugation to separate the unbound acidic mixture from the functionalized carbon nanotubes. Next, the purified carbon nanotubes may be washed before being combined with and being dispersed in the base fluid. The purified carbon nanotubes are then combined with the base fluid and dispersed substantially evenly in the base fluid through an ultrasonication operation. And during the ultrasonication operation, a cooling operation is applied for cooling a system performing the ultrasonication operation.

In one preferred embodiment, the functional groups are introduced onto the carbon nanotubes by treating with an acid mixture including H₂SO₄ and HNO₃ in a ratio of about 3:1 in a laboratory apparatus as shown in FIG. 1. The laboratory apparatus 1 comprises a beaker 10, a reflux system 11 coupled to the beaker 10, and a heating table 12. The mixture in the beaker 10 is heated and stirred over the heating table 12. As the liquid is heated over the boiling point to vaporize, the reflux system 11 condenses vaporized gas into liquid droplets and recycles them back into the beaker 10. Next, the functionalized carbon nanotubes may be purified by high speed centrifugation to separate the unbound acidic mixture from the functionalized carbon nanotubes. The purified carbon nanotubes may be washed with the base fluid before being combined with and being dispersed in the base fluid.

The ultrasonication operation is performed using a ultrasonic homogenizer 2 arranged adjacent to a dual tube heat exchanger 3 which efficiently dissipates the heat generated by the ultrasonic homogenizer 2 to cool the base fluid. Referring to FIG. 2, the ultrasonic homogenizer 2 comprises an ultrasonic probe 20 and a power supply 21 connected to the ultrasonic probe 20 for supplying power required for the ultrasonication operation. The ultrasonic probe 20 is arranged in such a way that a tip 20a of the ultrasonic probe 20 is dipped in the base fluid to effect dispersion. The dual tube heat exchanger 3 has an inner tube 30 in which the base fluid is received and an outer tube 31 surrounding the inner tube 30. The outer tube 31 is filled with a fluid to dissipate or carry away the heat generated by the ultrasonic probe 20. The outer tube 31 has an inlet 311 arranged at a bottom end and an outlet 312 arranged at a top end as shown in FIG. 2, such that the fluid enters the outer tube 31 via the inlet 311 and exits via the outlet 312 for cooling the ultrasonic homogenizer 2. So, the ultrasonication operation is not interrupted as a result of overheating the ultrasonic probe 20 with a substantially high power supplied to the ultrasonic probe 20 over a period of time. This ensures a constant output of high power during the ultrasonication operation to achieve an optimal dispersion effect.

In accordance with another preferred embodiment of the invention, the cooling system comprises the dual tube heat exchanger 3 as illustrated in FIG. 2 and a cooling circulation system 4. Referring to FIG. 3, the dual tube heat exchanger 3 is connected to cooling circulation system 4 in such a way that the fluid that flows out of the outer tube 31 is recycled via a pipe 40 back to the outer tube 31 for heat dissipation. And the pipe 40 may be connected to a cooling bath 41 for further cooling of the fluid in the pipe 40 before the fluid is re-directed back to the outer tube 31 of the dual tube heat exchanger 3. Thus, with the cooling system illustrated in FIG. 3, the fluid is efficiently recycled to provide heat dissipation or cooling for the ultrasonic homogenizer without wasting too much fluid in the cooling system. Therefore, the overall cost for preparing the nanofluid is effectively reduced.

It is noted that the cooling system in the present invention is not limited to the specific devices or instrumentalities as described in the above embodiments. For example, the cooling system may be modified or improved in the knowledge of those skilled in the heat exchange technique to achieve similar cooling effect on the ultrasonic probe and the ultrasonic homogenizer.

In light of the preparation methods described above, the present invention further provides a carbon nanofluid capable of serving as a heat transfer fluid. The carbon nanofluid comprises about 99.8 to about 98% by volume of a base fluid, and from about 0.2 to about 2.0% by volume of functionalized carbon nanotubes substantially evenly-dispersed in the base fluid, wherein the carbon nanofluid has a thermal conductivity at least 1.3 times higher than a base fluid having no carbon nanotubes.

The present invention further provides a carbon nanofluid made by the process of introducing a number of functional groups onto carbon nanotubes for providing functionalized carbon nanotubes, providing a base fluid, combining the functionalized carbon nanotubes with the base fluid, and dispersing the carbon nanotubes substantially evenly in the base fluid through an ultrasonication operation, and cooling a system performing the ultrasonication operation during the ultrasonication operation.

In accordance with the present invention, the base fluid includes but is not limited to organic solvents, inorganic solvent and aqueous solutions having carbon nanotubes substantially evenly dispersed therein for serving as a heat transfer agent. And depending on the actual application, the base fluid comprises at least one of ethylene glycol, water and oil. While the present invention also encompasses the carbon nanofluid mixed with surfactants or dispersants and the preparation method thereof, it is more preferable to prepare the fluid complex or carbon nanofluid without adding the surfactants or dispersants which will encapsulate or coat the carbon nanotubes to mask or reduce their high thermal conductivity.

The invention will now be described in further detail with reference to the following specific, non-limiting examples.

Preparation of Carbon Nanofluid

The carbon nanotubes of single-walled, double-walled or multiple-walled were commercially available (Nanotech Port Co., Shenzhen, China) and purchased in the form of powder. The carbon nanotubes were functionalized or surface modified by treating with an acidic solution including H₂SO₄ and HNO₃ in a ratio of about 3:1 in the laboratory apparatus illustrated in FIG. 1. Next, the functionalized carbon nanotubes were purified by high speed centrifugation to separate the unbound acidic solution from the functionalized carbon nanotubes. The purified carbon nanotubes were washed with the working fluid before dispersing the carbon nanotubes in the base fluid by ultrasonication.

An ultrasonication process was performed using an ultrasonic homogenizer in the presence of a cooling system, such as a dual tube heat exchanger shown in FIG. 3, capable of dissipating heat generated by the ultrasonication process. Therefore, as the carbon nanotubes were dispersed by ultrasonication in the base fluid, the heat generated by the ultrasonic probe could be dissipated instantly as a result of the fluid flowing through the outer tube. This ensured a stable operation of the ultrasonic homogenizer even if a high power of about 300 to 600 W was supplied to the ultrasonic probe over a period of time during the ultrasonication operation. Accordingly, a constant output of substantially high power was supplied during the ultrasonication operation to substantially evenly disperse the carbon nanotubes in the base fluid.

Thermal Conductivity Measurement

The thermal conductivity (k) of the carbon nanofluid was measured with specially designed, computer-controlled equipment as described (Lee et al., Journal of Heat Transfer, Vol. 121 pp. 280 (1999)). Thermal conductivities were measured as a function of nanotube volume fractions at room temperature. For the measurement of thermal conductivity, the carbon nanofluid was filled into the vertical, cylindrical glass container of transient hot wire system. The long glass container has an inner diameter of 19 mm and a length of 240 mm. In the transient hot wire system, a platinum wire having a diameter of about 76.2 μm was immersed in the carbon nanofluid. The platinum wire was simultaneously used as a heater and as an electrical resistance thermometer for the carbon nanofluid. The surface of platinum wire was coated with a thin electrical insulation epoxy for preventing the platinum wire from short circuitry. A temperature variation of the platinum wire was obtained as a result of change in the electrical resistance with time. The thermal conductivity was then estimated from Fourier's Law. The thermal conductivity of the carbon nanofluid was inversely proportional to the slope of the temperature versus time response of the platinum wire. The transient hot wire system was calibrated using deionized water and ethylene glycol at room temperature. Uncertainty of the measurement was less than 2%.

EXAMPLE 1 Nanofluid A (Carbon Nanotubes/Ethylene Glycol)

The nanofluid A was prepared by dispersing multiple walled carbon nanotubes in ethylene glycol. No surfactant was added to the nanofluid A. And the carbon nanotubes were combined with and dispersed in ethylene glycol through an ultrasonication operation at 600 W for approximately one hour. During the ultrasonication operation, a cooling operation was applied using a dual tube heat exchanger as shown in FIG. 2 for cooling the nanofluid A during the ultrasonication operation performed by an ultrasonic homogenizer.

Next, the nanofluid A was subjected to thermal conductivity measurement as described above. As listed in Table 1 below, the thermal conductivity (represented by k value) was increased by 12.4% at a volume fraction of 0.01 (1 vol. %) for the carbon nanotubes/ethylene glycol suspensions as compared with ethylene glycol only. Therefore, small amount of carbon nanotubes dispersed according to the present invention resulted in a significant increase in the thermal conductivity of the base fluid. TABLE 1 carbon nanotubes/ethylene glycol vol. % k value increase (%) 0.2 1.6 0.4 3.6 0.5 7.6 1.0 12.4

EXAMPLE 2 Nanofluid B (Carbon Nanotubes/Water)

The nanofluid B was prepared by dispersing multiple walled carbon nanotubes in the water. No surfactant was added to the nanofluid B. And the carbon nanotubes were combined with and dispersed in the water through an ultrasonication operation at 600 W for approximately one hour. During the ultrasonication operation, a cooling operation was applied using a dual tube heat exchanger as shown in FIG. 2 for cooling the nanofluid B during the ultrasonication operation performed by an ultrasonic homogenizer.

Next, the nanofluid B was subjected to thermal conductivity measurement as described above. As listed in Table 2 below, the thermal conductivity was increased by 17.8% at a volume fraction of 0.015 (1.5 vol. %) for the carbon nanotubes/water suspensions as compared with water only. Therefore, small amount of carbon nanotubes dispersed according to the present invention resulted in a significant increase in the thermal conductivity of the working fluid. TABLE 2 carbon nanotubes/water vol. % k value increase (%) 0.25 2.6 1.0 9.5 1.5 17.8

EXAMPLE 3 Nanofluid C (Carbon Nanotubes/Synthetic Engine Oil)

The nanofluid C was prepared by dispersing multiple walled carbon nanotubes in the synthetic engine oil. N-hydroxysuccinimide (NHS) was added to the nanofluid C. And the carbon nanotubes were combined with and dispersed in the synthetic engine oil through an ultrasonication operation at 600 W for approximately one hour. During the ultrasonication operation, a cooling operation was applied using a dual tube heat exchanger as shown in FIG. 2 for cooling the nanofluid C during the ultrasonication operation performed by an ultrasonic homogenizer.

Next, the nanofluid C was subjected to thermal conductivity measurement as described above. As listed in Table 3 below, the thermal conductivity was increased by 30.3% at a volume fraction of 0.02 (2.0 vol. %) for the carbon nanotubes/synthetic engine oil suspensions as compared with oil only. Therefore, small amount of carbon nanotubes dispersed according to the present invention resulted in a significant increase in the thermal conductivity of the working fluid. TABLE 3 carbon nanotubes/synthetic engine oil vol. % k value increase (%) 1.0 8.5 2.0 30.3

Although in the above embodiments, it is understood by one skilled in the art that other base fluids having carbon nanomaterial dispersed therein, whether functionalized or not, may also be within the scope of the invention in view of the carbon nanofluid and preparation method described in the present invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

It be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 

1. A method for preparing carbon nanofluid, the method comprising: providing a base fluid; providing a number of carbon nanotubes; combining the carbon nanotubes with the base fluid; and dispersing carbon nanotubes substantially evenly in the base fluid through a physical agitation operation; and cooling a system performing the physical agitation operation during the physical agitation operation.
 2. The method according to claim 1, wherein the physical agitation comprises ultrasonication.
 3. The method according to claim 1, wherein the carbon nanotubes comprise at least one of single-walled, double-walled and multi-walled carbon nanotubes having a plurality of functional groups introduced thereon.
 4. The method according to claim 3, wherein each of the functional groups comprises COOH.
 5. The method according to claim 1, wherein the base fluid comprises at least one of ethylene glycol, water and oil.
 6. A method for preparing a fluid capable of serving as a heat transfer agent, the method comprising: introducing a number of functional groups onto carbon nanotubes for providing functionalized carbon nanotubes; providing a base fluid; combining the functionalized carbon nanotubes with the base fluid; and dispersing the carbon nanotubes substantially evenly in the base fluid through an ultrasonication operation; and cooling a system performing the ultrasonication operation during the ultrasonication operation.
 7. The method according to claim 6, wherein introducing the functional groups comprises treating with an acidic solution comprising H₂SO₄ and HNO₃ in a ratio of about 3:1
 8. The method according to claim 7, further comprising purifying the functionalized carbon nanotubes by high speed centrifugation before combining the carbon nanotubes with the base fluid.
 9. The method according to claim 8, wherein the carbon nanotubes comprise at least one of single-walled, double-walled and multi-walled carbon nanotubes.
 10. The method according to claim 8, wherein each of the functional groups comprises COOH.
 11. The method according to claim 8, wherein the base fluid comprises at least one of ethylene glycol, oil and water.
 12. A carbon nanofluid capable of serving as a heat transfer fluid, the carbon nanofluid comprising: (a) about 99.8 to about 98% by volume of a base fluid; and (b) from about 0.2 to about 2.0% by volume of functionalized carbon nanotubes substantially evenly-dispersed in the base fluid, wherein the carbon nanofluid has a thermal conductivity at least 1.3 times higher than a base fluid having no carbon nanotubes.
 13. The carbon nanofluid according to claim 12, wherein the functionalized carbon nanotubes comprise at least one of single-walled, double-walled and multi-walled carbon nanotubes with a number of functional groups introduced thereon.
 14. The carbon nanofluid according to claim 12, wherein the base fluid comprises at least one of ethylene glycol, water and oil.
 15. A carbon nanofluid made by the process of: introducing a number of functional groups onto carbon nanotubes for providing functionalized carbon nanotubes; providing a base fluid; combining the functionalized carbon nanotubes with the base fluid; and dispersing the carbon nanotubes substantially evenly in the base fluid through an ultrasonication operation; and cooling a system performing the ultrasonication operation during the ultrasonication operation. 